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The Gram stain is a general stain used extensively in microbiology for the preliminary differentiation of microbiological organisms. The Gram stain is 1 of the simplest, least expensive, and most useful of the rapid methods used to identify and classify bacteria.
The Gram stain is used to provide preliminary information concerning the type of organisms present directly from clinical specimens or from growth on culture plates. This stain is used to identify the presence of microorganisms in normally sterile body fluids (cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid). It is also used to screen sputum specimens to establish acceptability for bacterial culture (<25 squamous epithelial cells per field is considered an acceptable specimen for culture) and may reveal the causative organism in bacterial pneumonia.
This information is useful in guiding initial antimicrobial therapy.
Gram-negative organisms do not retain crystal violet or iodine which are subsequently washed out by acetone/alcohol. Subsequently, they appear red because they are only stained with safranin dye. On the other hand, gram-positive organisms retain the crystal violet iodine complex after decolorization with acetone/alcohol and, therefore, stain purple.
Over decolorization may result in the loss of the crystal violet iodine complex from gram-positive organisms and result in a misinterpretation.
No organisms seen or descriptive report of observations.
Forbes BA, Sahm DF, Weissfeld AS: Bailey and Scott’s Diagnostic Microbiology. 11th edition. Mosby-Elseiver, St. Louis, MO 2002, pp 122-125